Directive antenna system



20, 1945- N. E. LINDENBLAD 0,

DIRECTIVE ANTENNA SYSTEM Filed D60. 31, 1940 INVENTOR NILS 5. LINDENBLADBY M'z/(A/ ATTORNEY Patented Feb. 20, 1945 DIRECTIVE ANTENNA SYSTEM NilsE. Llndenblad, Rocky Point, N.'Y., aslignor to Radio Corporation ofAmerica, a corporation of Delaware Application December 31, 1940, SerialNo. 372,609

9 Claims.

The present invention relates to an antenna system for ultra-highfrequency radiation and, more particularly, to antenna systems employingparabolic reflectors. for projection of the radiation in a desireddirection.

It has previously been attempted to conserve the direct forwardradiation from a radiator at the focus of a parabolic reflector by meansof a spherical cap over the radiator. This method yields an improvementin directivity but also results in an increase in' the resonancecharacteristic or the Q of the radiation system. This is very seldomdesirable in modern wide band frethe order of a wavelength in diameteror less at a distance less than a half wavelength in front of theradiator.

According to another aspect of my invention, I also propose to improvethe fleld distribution across the reflector by placing a conductive conebetween the radiator within the reflector and the portion of thereflector immediately back of the radiator. My invention will be morefully understood by reference to the following detailed descriptionwhich is accompanied by a drawing in which reference character iindicates a parabolic reflector having mounted therein at the focus aradiating antenna structure 2. A portion of the reflector i is shownbroken away in order that the interior arrangement may be clearly seen.The antenna structure 2 is connected to energizing or signal transducermeans (not shown) by a transmission line TL. Immediately in front of theradiator 2 is placed a conductive disc or shading element 3 whichpreferably has a diameter of the order of one wavelength and is placed adistance of a half the operating wavelength or less in front of radiator2. The distance between the radiator 2 and the conductive disc 3 is notcritical but in general it will be somewhat less than a half of theoperating wavelength. The presence of the disc-like shading element 3prevents the direct radiation of energy from radiator 2 and thusimproves the radiation pattern of the antenna ystem in the same way asdo the spherical caps previously known in the art.

However, the flat conductive disc 3 does not change the resonancecondition of the radiator to nearly the same extent as the sphericalcaps due to sectional variation of distance to radiator.

In a reflector of practical parameter, the distance from the radiator 2to the reflecting surface of the reflector I increases considerablytowards the edge of the reflector. I have discovered that this conditionresults in a poor field distribution across the face of the reflector sothat all the gain that is theoretically possible to obtain with acertain reflector area is not obtained. I find that it is possible tocorrect this condition by the use of a centrally located cone 5, asindicated in the accompanying drawing. A portion of'reflector i has beenshown as broken away in order to more clearly disclose cone 5. Thepresence of this cone does not materially affect the resonance conditionof the radiator. As indicated in the drawing, the included angle of thecone 5 should preferably be of the order of and the length of the sideshould be of the order of a half wavelength, though this is notcritical.

The theoretical formula for the gain of a reflector having an openingarea A expressed in terms of the wavelength squared and when usinguniform power distribution is Gain=K x A where K is equal to 7.8. Thoseskilled in the art have heretofore found that it is difflcult to exceedpractically a value of 4 for the constant K as compared to thetheoretical value of 7.8.

By utilizing the flat conductive disc 3 and the centrally located cone 5in accordance with the teachings of my invention, it is possible toobtain a value for K of 5.6, or more, which represents a conservativeinterpretation of actual measurements on a physical embodiment of myinvention. It will be seen that this is a substantial improvement overwhat has previously been obtainable. My invention has been described indetail with reference to a reflector in the form of a parab oloid ofrevolution and a conical angle limiting structure but it should beclearly understood that the same principles may be used with a reflectorhaving the form of a cylindrical parabola and a prismatic angle limitingstructure.

While I have particularly shown and describ d several modifications ofmy invention, it is to be distinctly understood that my invention is notelement disposed across the opening of said reflector in front of saidradiating means at a distance less than half the operating wavelengththerefrom and means interposed between said reflector and said radiatingmeans for re-distributing the field within said reflector for mosteffective use of the surface of said reflector.

2. An antenna system comprising radiating means within a parabolicreflector, a flat shading element disposed across the opening of saidreflector in front of said radiator means at a distance less than halfthe operating wavelength therefrom and means interposed between saidreflector and said radiating means for limiting the effective surface ofsaid refle tor, said last means comprising a right circular cone.

3. An antenna system comprising radiating means within a parabolicreflector, a flat shading disc disposed across the opening of saidreflector in front of said radiator means at a distance less than halfthe operating wavelength therefrom and means interposed between saidreflector and said radiating means for limiting the effective surface ofsaid reflector, said last means comprising a right circular cone havinga side equal in length to a half of the operating wavelength.

4. An antenna system comprising radiating means within a parabolicreflector, a flat shading disc disposed across the opening of saidreflector in front of said radiator means at a distance less than halfthe operating wavelength therefrom and means interposed between saidreflector and said radiating means for re-distributing the fleld withinsaid reflector for most effective use of the surface of said reflector,said last m ans comprising a right circular cone having a side equal inlength to a half of the operating wavelength.

5. An antenna system comprising radiating means within a parabolicreflector and means interposed between said reflector and said radiatingmeans for distributing the fleld within said reflector for a mosteffective use of the surface of said reflector, said means comprising aconductive cone, the apex of said cone facing said radiating means, saidcone having a side length of the order of one-half of the operatingwavelength of said system.

6. An antenna system comprising electromagnetic radiating means within aparabolic reflector and means interposed between said radiating meansand a portion of said reflector for limiting the distribution ofradiated energy to predetermined portions of said reflector, said meanscomprising a conductive body tapering from said reflector toward saidradiating means, the length of the tapering side of said body being ofthe order of one-half of the operating wavelength of said system.

7. The combination with a radio frequency radiator and wave directivestructure for concentrating radiation from said radiator in a givendirection of means interposed between said radiator and a portion ofsaid structure for limiting distribution of radiated energy from saidradiator to predetermined portions of said structure, said meansincluding a conductive body tapering from said structure toward saidradiator the length of said conductive body being of the order ofone-half of the operating wavelength of said radiator.

8. An antenna system comprising radiator means within a parabolicreflector and means interposed between said reflector and said radiatormeans for distributing the field within said reflector for a mosteffective use of the surface of said reflector, said means comprising aright circular conductive cone, the apex of said cone facing saidradiator means, said cone having a side length of the order of one-halfof the operating Wavelength of said system.

9. An ant nna system including radiating means within a parabolicreflector, a flat shading element disposed across the opening of saidreflector in front of said radiating means, and means interposed betweensaid reflector and said radiating means for re-distributing the fleldwithin said reflector for the most effective use of the surface of saidreflector, said last-mentioned means including a conductive cone, theapex of said cone facing said radiating means.

NILS E. LINDENBLAD.

